War crimes are always perpetrated by the loser in war. Though both sides may commit crimes, the victors have always been able to turn might into right, ignoring their own violations and penalising their enemy. At Nuremberg in 1945, the western states knew that their bombing of German cities could pose awkward questions and they quietly dropped their charges against the Luftwaffe; the democracies sat side-by-side with the Soviet Union, which many people argued at the time could itself be regarded as guilty on several of the same counts for which German leaders were indicted.

Should Saddam Hussein be caught alive, he will be made to account for years of crimes against humanity, if he is not murdered first by trigger-happy US forces. Western consciences will have no problems about arraigning Saddam and his henchmen. They will be expected to pay the way Hitler and his gang were expected to pay in 1945, though it is worth remembering that until a trial was finally agreed on in May 1945, Churchill preferred the idea that Nazi leaders should be shot on the spot once they were captured. Saddam might join Milosevic at the Hague, as a warning to tyrants worldwide that a grim justice awaits them.

But this time the situation is different. The legal position is anything but clear-cut. A good deal of informed opinion worldwide regards the Anglo-American invasion and conquest of Iraq as an illegal act of aggression, in the course of which it is coalition forces that have perpetrated numerous war crimes while pulverising Iraqi resistance. The Nuremberg precedent might be invoked to argue that committing crimes in order to overcome tyranny is legally permissible, but there is an awkward contrast with the treatment of German war crime in 1945: now it is the US and Britain that many believe have waged a war of aggression.

It is not difficult to imagine how the case for the prosecution against the coalition might be constructed. An indictment would have three main elements. In the first place, Britain and the US have waged an illegal war, without the sanction of a UN resolution (in itself of dubious legality when it comes to a war launched in violation of the UN charter and fought on this scale). Any argument that Saddam's failure to disarm fast enough justified the invasion of his state, the destruction of Iraq's major cities and the killing of thousands of Iraqis fails on the legal concept of proportionality. In British law, a householder may not cut an intruder to shreds with an axe on suspicion of burglary; if he does so, he becomes the object of prosecution. The suspected - but as yet unproven - violations of disarmament resolutions should not justify in international law the massive destruction and dislocation of the entire Iraqi state.

Ironically, the one instrument the Allies could find in 1945 to explain that Hitler's wars were illegal was the Kellogg-Briand pact, signed in Paris in 1928 at the behest of the then American secretary of state. The pact had outlawed war as an instrument of policy for all the signatory powers, including Britain and the US, but its precise status in international law was open to dispute. At Nuremberg, the American chief prosecutor, Justice Jackson, insisted on using it as the foundation for the whole case against Hitler. It could still be the foundation of the case against British and American belligerence.

The second and third elements of any prosecution derive not only from the initial presumption that the coalition has waged an illegal war. As at Nuremberg, the subsequent killing of civilians and mistreatment of prisoners in a war of aggression also constitute war crimes in their own right. No legal niceties are needed to see that the American and British killing of thousands of Iraqi civilians could be approached in this way. The mistreatment of Iraqi prisoners of war dwarfs the brief appearance of US servicemen on Iraqi television. Pictures of stripped and bound prisoners have already been released. The camps constructed early in the campaign were closed to the Red Cross in defiance of the Geneva convention. If prisoners are subsequently taken to the US and subjected to the same treatment as the Afghan soldiers held at Guantanamo Bay, this too would be a violation of international law.

The sad truth is that prosecution has always been a function of power. No one seriously believes that Bush and Blair will be indicted. International law works only against weaker states. Big powers have an unmerited, but unassailable, immunity. Even if anyone were brave or rash enough to try to indict coalition leaders, the US has refused to ratify the statute establishing the international criminal court, which came into force on July 2 2002.

The court has been set up to deal with gross violations of international law and human rights. Technically it can prosecute state nationals from states that have not subscribed to the statute. But the view has been widely held that the US refused to join because it wanted to be able to dish out its own justice. The American declaration that it intends to take prisoners back to the US for trial opens up the prospect that there will be one law for the criminal court, if Britain were ever to be indicted, and one for America. The absence of a commonly agreed jurisdiction could invalidate the whole enterprise and confirm the fact already evident that political power, not justice, will determine the future.

The operation of double standards has been evident throughout the campaign. What the coalition does with impunity is hailed as a war crime when it is committed by Iraqis. The image of crude American gun law, evident in the efforts to kill Saddam, has been justified by American international lawyers. In the unlikely eventuality that either Bush or Blair are blown up or shot, there would be outrage. Yet, on any reckoning, it should be entirely legal, if it is legal to murder Saddam. The coalition cannot have it both ways.

There is no prospect that Bush and Blair will be sharing a cell with Saddam at the Hague. The death and destruction meted out in their name will have to lodge instead in their consciences. For the rest of the world, the prospect is an unattractive one. The appearance of lawlessness, promoted by those very states which should be among the first to demonstrate their commitment to international standards, will provoke further lawlessness, first in Iraq, then perhaps throughout the Middle East.

In international affairs, lawless behaviour is unaccountable, which is why at Nuremberg efforts were made to find some measure by which such things could be brought to account. There is now no means through which the international community can restrain American power, nor its pale British shadow. The last three weeks of coalition violence have destroyed 60 years of patient international collaboration to build a sound framework for the conduct of affairs between states. Justice Jackson must be turning in his grave.

· Richard Overy is the author of Interrogations: Inside the Mind of the Nazi Elite and Russia's War, both published by Penguin